1. Field of the Invention
The present invention relates to radiographic apparatuses.
2. Description of the Related Art
Methods of producing a radiograph of a subject by applying radiation to the subject and detecting the intensity distribution of radiation passing through the subject are widely and generally used in industrial nondestructive testing and medical diagnosis. A specific example of a general method of producing a radiograph of a subject employs a combination of a so-called “fluorescent screen” (or intensifying screen) that emits fluorescent light when excited by radiation and a silver film. Radiation is applied to the subject, and rays that pass through the subject are converted by the fluorescent screen into visible light. The silver film is exposed to this visible light, thus forming a latent image on the silver film. The silver film is then subjected to chemical processing to produce a visible image. A radiograph produced by this method is an analog photograph for use in diagnosis, testing, etc.
A computed radiographic apparatus (which hereinafter may also be referred to as a CR apparatus) that employs an imaging plate with an induced phosphor layer (hereinafter referred to as an IP) has become widely used. The IP, which is primarily excited by application of radiation and secondarily excited by visible light by, for example, a red laser, produces induced phosphorescent light. The CR apparatus detects this light emission using an optical sensor, such as a photomultiplier, to produce a radiograph. On the basis of this image data, the CR apparatus outputs a visible image to a photographic material, CRT, etc. Although the CR apparatus is a digital apparatus, the CR apparatus requires an image formation process in which an image is read by secondary excitation and is thus an indirect digital radiographic apparatus. “Indirect” here means that a captured image cannot be displayed simultaneously, as in analog technology.
Technology has been developed for producing a digital image by using, as an image receiver, a photoelectric transducer with a matrix of pixels including micro photoelectric transducer elements, switching elements, and the like. An image capture apparatus based on this technology can simultaneously display produced image data and is thus referred to as a direct digital image capture apparatus. Such a digital image capture apparatus affords an advantage over analog photographic technology in that the former needs no film and can emphasize produced information by image processing and put such information in a database. The direct digital image capture apparatus has an advantage of immediateness over the indirect digital image capture apparatus. Whereas the indirect type requires an image formation process including secondary excitation, the direct type converts a radiograph into digital data immediately after the image is captured. Whereas the indirect type requires an additional reader for secondary excitation, the direct type requires no such component.
Since an image capture apparatus using known silver photography has a narrow dynamic range with respect to the amount of radiation received, exposure is often incomplete or excessive. To ensure stable exposure, a radiation detector is placed in front of or behind the film. The image capture apparatus employs an automatic exposure control (AEC) circuit, which is referred to as a phototimer or the like, that integrates the output of the radiation detector, compares the integral with a preset value determined to achieve necessary film blackness for diagnosis, and, when the integral reaches the preset value, transmits an X-ray cut-off signal to cut off the X-ray exposure.
A digital image capture apparatus is advantageous over known silver photography in that the former has a wider dynamic range. Compared with silver photography, the digital image capture apparatus is more tolerant to incomplete or excessive exposure. Even when the amount of radiation reaching a test subject is inappropriate, the digital image capture apparatus can produce an image output suitable for diagnosis by performing image processing, such as gray level transformation or the like. When the amount of radiation reaching a test subject is low, as in general silver photography, the influence of quantization noise in the radiation intensity distribution and system noise of the apparatus becomes greater, thus degrading the S/N ratio of the image. To have a minimum amount of radiation reaching a test subject so that the minimum required quality of a produced image can be achieved, an AEC circuit is used to perform AEC, as in silver photography. U.S. Pat. No. 5,448,613 describes an X-ray diagnostic apparatus including an X-ray image intensifier and a semiconductor detector including pixels arranged in a matrix, in which AEC is performed using the outputs of some of the pixels.
A medical radiograph of a test subject is taken using a scattered ray absorption member referred to as a grid, which is placed between the test subject and the detector, to reduce the influence of scattered rays generated when radiation passes through the test subject. Generally, the grid has an array of foils such as lead foils, through which almost no radiation passes, and foils such as aluminum foils, through which radiation easily passes, which are alternately disposed in a direction orthogonal to the direction in which radiation is applied. With this arrangement, most of the radiation scattered from the test subject is absorbed in the lead foils of the grid before reaching the detector. As a result, a high-contrast image is produced.
The mentioned known digital image capture apparatus of U.S. Pat. No. 5,448,613 with a flat panel detector (FPD) using solid-state optical detectors includes an AEC circuit having radiation detectors disposed in front of the FPD, the radiation detectors being separate from the FPD. To satisfy size reduction, simplification, and cost reduction requirements for the apparatus, on the basis of advances in manufacturing technology, it has been proposed to arrange AEC radiation detectors between pixels in the FPD. In this case, the AEC radiation detectors are arranged advantageously in a stripe pattern between pixels in the FPD in order to simplify FPD read-out driving control, to minimize the influence on peripheral pixels, and to avoid as much image quality degradation as possible.
When the apparatus is used in conjunction with the grid, the AEC radiation detectors arranged in a stripe pattern are hidden behind the lead foils of the grid. As a result, the amount of rays cut off by the AEC circuit (cumulative amount of radiation) may be wrong.